The present invention relates to a method for fabrication of thin films. More specifically, the invention relates to a method for fabricating on a semiconductor substrate a thin film which is uniform in resistivity.
When a thin film such as a silicon semiconductor single crystal thin film having a specified resistivity is fabricated on a semiconductor substrate (hereinafter, referred to simply as "substrate" from time to time) such as a silicon semiconductor single crystal substrate, it is practiced to take the steps of, with the semiconductor substrate placed in a reaction vessel of vapor phase growth equipment, supplying a raw material gas for growth of a thin film as well as a dopant gas through a gas inlet provided in the reaction vessel, and increasing the temperature of the semiconductor substrate higher than the reaction temperature of the raw material gas, by which a thin film doped with dopant elements is deposited and formed on the semiconductor substrate.
As vapor phase growth equipment, conventionally, there have been available those called "hot wall" type in which the inside wall of the reaction vessel is not cooled but left at a temperature nearly equal to the substrate temperature, and those called "cold wall" type in which the inside wall of the reaction vessel is cooled so as to be kept lower than the substrate temperature.
In the hot wall type vapor phase growth equipment, because of the high inside-wall temperature of the reaction vessel, easily thermally-decomposable raw material gas for thin film growth and dopant gas, when introduced into the hot wall type reaction vessel, decompose before reaching the substrate surface, so that a substantial portion of the gases is deposited on the inside-wall surface of the reaction vessel, and thus consumed. Therefore, for the reason that the raw material gas for thin film growth and the dopant gas do not reach the substrate surface sufficiently, there have been issues that film thickness distribution of the thin film to be formed is difficult to control or resistivity distribution within the thin film becomes nonuniform and difficult to control. Whereas the issue of the control of film thickness distribution is beginning to be solved by using a reaction atmosphere under reduced pressure or other means, no effective solutions have been developed for the issue of resistivity distribution.
In the case of the cold wall type vapor phase growth equipment, on the other hand, it is known that because the inside-wall temperature of the reaction vessel is set to about 500.degree. C., which is lower than the thermal decomposition temperature of the raw material gas for thin film growth, deposition of the thin film onto the inside wall of the reaction vessel is suppressed, which produces an effect of preventing losses of the raw material gas for thin film growth. However, attentions have not been paid at all hitherto to losses of the dopant gas due to its thermal decomposition at the inside wall of the reaction vessel. Accordingly, there has been an issue that the resistivity distribution of the thin film may become difficult to control due to any inappropriate setting of the inside-wall temperature of the reaction vessel.
Also, thermal decomposition of the dopant gas at the inside wall of the reaction vessel would cause dopant elements to be deposited on the inside wall of the reaction vessel. Therefore, if the thin film continues to be fabricated without removing the deposit by etching, the deposited dopant elements would be liberated from the inside-wall surface of the reaction vessel during the reaction and mixed into the thin film. This would lead, in some cases, to occurrence of the so-called memory effect that a higher than designed concentration level of dopant is taken into the growing thin film even if a specified level of dopant gas is introduced into the reaction vessel. This memory effect has been a factor that makes it difficult to control the resistivity distribution of thin films among different substrates.